From reading on the USPTO website, a trademark seems to go through a standard series of stages: filing, examination, publication, registration.
At what point can the individual who filed the trademark sue someone for infringing it; the filing date? The final registration date?
If a hypothetical company was infringing this trademark for a period of time after the filing date, but before the final registration date they ceased completely, could this company be sued?
You have a trademark before it is registered, even before the registration process is started, by the fact that you are using the name. Therefore you can sue at any time. The defendant will find it easier to avoid damages because they were not aware of your trademark. The plaintiff will find it harder to prove the existence of the trademark. But in principle, you can be sued at any time.
If you are aware that someone filed for a trademark, unless you have good reasons to believe that the filing will be refused (for example because you object to it), don't use that trademark.
The owners of a trademark may enforce its inherent rights in state or federal courts from the moment they have used it as a distinctive brand on goods or services in commerce. Registration in the USA is completely optional but serves to establish a "preemption priority date" as of the date of filing, as pertinent to the state or federal statutes for registration. The owners obviously have the burden of proving damages arising from the infringing use or proving a basis for injunctive relief.
